The invention relates to a semiconductor memory device and, more particularly, to a voltage regulator for a semiconductor memory device.
Semiconductor memory devices are storage devices that contain data therein and read out the stored data therefrom. Semiconductor memory devices are generally classified into random access memory (RAM) and read only memory (ROM). RAM is a volatile memory device that loses data stored in its memory cells when electrical power supplied to the device is interrupted or suspended. ROM is a nonvolatile memory device that retains data in its memory cell even when the electrical power supplied to the device is shut down. ROM includes various kinds such as a programmable ROM (PROM), an erasable PROM (EPROM), an electrically EPROM (EEPROM), and a flash memory device.
The semiconductor memory device includes a voltage regulator to supply a target voltage of a constant level into an internal circuit. For example, Korean Patent No. 10-0362700 (U.S. Pat. No. 6,442,079, issued to Byeong-Hoon Lee, et al. on Aug. 27, 2002) discloses a voltage regulator for nonvolatile storage devices of an electrically erasable and programmable semiconductor type. As generally known in the prior art such as the aforementioned U.S. patent, a conventional regulator includes a comparator, a PMOS transistor used as a driver, and resistors used as a voltage dividing circuit. The comparator is composed of a differential amplifier and discriminates whether an output voltage of the voltage dividing circuit is lower than a reference voltage. The PMOS transistor operates according to the discriminated result of the comparator. For example, when an output voltage of the voltage regulator is lower than a target voltage, the comparator turns on the PMOS transistor, causing an increase of an output voltage level. In contrast, when the output voltage of the voltage regulator is higher than the target voltage, the comparator turns off the PMOS transistor, causing a decrease of the output voltage level.
However, in the conventional voltage regulator, it takes a long time for the output voltage to reach the target voltage. Particularly, when the target voltage is lower than 1 V, a significant problem occurs. When the target voltage is lower than 1 V, the reference voltage also becomes lower than 1 V. At this time, the reference voltage can become almost identical with a threshold voltage of an NMOS transistor in a differential amplifier. When the reference voltage becomes almost identical with the threshold voltage of an NMOS transistor, the time required that the comparator discharges a gate of a PMOS transistor is increased. Accordingly, the PMOS transistor is turned on later, resulting in an increased setup time of the target voltage.